👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @Clorith! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

I am still curious why the removal of the menu item simply can't be reverted.

Adding a quick note that my patch here implements a new function, gutenberg_site_requires_customizer(), although we could just as easily just add in an if statement for has_action() into the two locations that rely on it, this was done in case it needed to be more flexible (for example if strange edge cases needed to be accounted for as well, making it easier to add them in one location right away).

As for the discussion about whether something should be reverted or not, I'll leave that for the product team to tackle, my main concern with this PR was to ensure backwards compatibility and nothing more :)

Thanks for working on this!

I think we should strive to implement this PR as the solution to #35874. If possible I'd prefer to not always show a link to the Customizer as I feel it will be confusing to users who are brand new to WordPress to have both an Appearance → Editor and Appearance → Customize.

We may end up having to always show a link to the Customizer. For example, if we don't end up with a solution for #29126 in time for WP 5.9 then we don't really have a choice. We can make this decision in the 5.9 beta period if need be.

Sorry @Clorith, looks like there are more conflicts. Could you please rebase again? Feel free to merge once resolved.